When scrap metal is heated to a liquid, molten state, certain impurities may be separated from the molten metal by the introduction of conventional fluxes which react with the impurities to form what is conventionally known as furnace slag. This slag rises to the surface and floats on top of the molten metal.
Slag is of little or no value in making use of the molten metal from the furnace. To the contrary, furnace slag can interfere with alloy additives in various metal specifications.
For example, in making alloyed steel, soluble oxygen is an unwanted contaminant. Slag which rises to the top of molten steel contains a large amount of soluble oxygen. If slag is present when alloys are added to the molten steel, then the soluble oxygen in the slag will react with the alloys and inhibit the alloys from reacting with the molten steel. Thus, the slag inhibits the alloying process. Also, the presence of slag in the molten steel facilitates the formation of particulate inclusions which, if large enough, may be detrimental to the physical properties of the steel.
Since furnace slag is a contaminant which may have a deleterious effect on making alloy steels, it is desirable to separate the slag from the molten metal before alloys are added to the molten metal. Therefore, slag separation is usually effected before alloys are added to the molten steel. Any slag which is separated is usually discarded. The process of separating slag from molten steel is often known as slag control.
Slag control has been a particularly difficult problem when scrap steel is melted in tilting furnaces and then discharged into a container or "ladle" before adding alloys. As discussed below, there have been numerous attempts at separating slag from molten steel that is discharged from a tilting furnace.
The typical electric furnace is mounted on a tilting platform. A tap hole is located on the side of the furnace. A discharge trough is mounted on the side of the furnace, just below the tap hole.
When the furnace is heated, scrap steel in the furnace melts into a molten liquid state. Slag separates from the molten steel and floats in a separate layer on top of the molten steel.
The tap hole is opened when the furnace is in the upright position. When the tap hole is opened, it is usually located above the level of the floating slag and molten metal. However, in some cases, it may be located below the level of the floating slag.
When the furnace is tilted, the operator of the furnace will attempt to tilt the furnace sufficiently so that the tap hole is below the top of the molten metal and permits the molten steel to flow through the tap hole. The slag remains inside the furnace and floats at a level above the level of the tap hole. As the molten steel drains from the furnace, the operator increases the angle of tilt in order to keep the slag at a level above the level of the tap hole. Thus, the operator attempts to cause all of the molten steel to flow through the tap hole before the slag begins to flow through the tap hole. This process of pouring or tapping is conventionally known as the "tap".
As slag floats on top of molten steel, there is a very fluid layer of floating slag, known as interface slag, which floats in a layer between the molten steel and the rest of the floating slag. The interface slag has much less viscosity, and a higher concentration of soluble oxygen, than the rest of the floating slag. Interface slag is particularly deleterious to the alloying process.
While molten steel is flowing through the tap hole, a vortex forms. The vortex draws interface slag through the tap hole while the molten steel is flowing through the tap hole.
The operator cannot see the vortexing of the interface slag because the furnace is usually enclosed on all sides and the top. Therefore, there is very little that the operator can do to prevent the interface slag from contaminating the molten steel during the tap.
During the tap, the level of the molten metal and floating slag in the furnace falls until the floating slag is at the level of the tap hole. At this point, the floating slag will begin to flow through the tap hole and contaminate the molten steel which has already been poured from the furnace. In order to prevent the flow of slag through the tap hole, the operator attempts to stop the tapping process quickly by closing the tap hole and/or returning the furnace to the upright position.
However, because a tilting furnace is usually fully enclosed, the operator usually cannot see inside the furnace to determine exactly when the slag is about to flow through the tap hole. Therefore, the operator usually waits until he sees slag coming out of the tap hole and into the trough before attempting to stop the flow of slag and returning the furnace to the upright position. This is the traditional method of slag control in a tilting furnace.
There have been numerous attempts to supplement or improve this basic method of slag control on tilting furnaces, including the use of tap hole gates, Vost-Alpine slag stoppers, the E-M-L-I system, and various stopper devices or plugs.
Tap hole gates are sliding or rotary gates which are mounted on the outside of the furnace adjacent the tap hole. The operator closes the gate when slag begins to discharge from the tap hole.
The Vost-Alpine slag stopper is a large, articulating nitrogen gas cannon which is used to close the tap hole. Operating under very high pressure, the cannon discharges nitrogen gas into the tap hole of the furnace on demand, and this stops the flow of molten steel and slag through the tap hole. Thus, the Vost-Alpine slag stopper is functionally analogous to a tap hole gate.
The E-M-L-I system consists of an electronic sensor which is mounted to the furnace inside the tap hole refractory. The E-M-L-I senses when a predetermined percentage of slag is entrained in the molten metal which is flowing through the tap hole. When the predetermined percentage is sensed by the E-M-L-I unit, the sensor communicates this to the operator of the furnace, who will then return the furnace to the upright position. Thus, the E-M-L-I system is used to control slag by directing the operator of the furnace to stop flow through the tap hole as soon as a predetermined amount of slag begins to flow through the tap hole.
A variety of stopper devices or plugs are used to control slag. They have a variety of shapes including the shapes of a tetrahedron or globe (also known as "cannonball"). A plug is placed inside the furnace and floats in the interface between the molten metal and floating slag. When the interface and plug drop to the level of the tap hole during the course of a tap, the plug is drawn by suction to the tap hole and blocks flow through the tap hole.
The eccentric bottom tapping gate is another attempt at slag control in an electric arc furnace. It requires that the tap hole be made in the bottom, rather than the side, of the furnace. When the operator observes slag pouring from the furnace, he closes a sliding gate to block the tap hole and prevent further flow through the tap hole. This method of slag control is quite expensive because it requires modification of an existing furnace to create a virtually new furnace and new ladle transfer cars or turrets to receive the molten steel as it is discharged from the furnace. The ladles must be moved from the side of the furnace and placed underneath the bottom of the furnace.
None of these prior methods of slag control for a tilting furnace have performed particularly well. None of them solves the problem of contamination of the molten steel with interface slag which vortexes through the tap hole while the molten steel is flowing through the tap hole. None of them solves the problem of contamination of the molten steel with slag which flows through the tap hole at the end of a tap before the operator can react to stop the flow through the tap hole. Most of these methods also stop the flow of some of the molten steel, thus reducing the yield.
The above-described prior art methods and apparatus do not control slag after it escapes through the tap hole of a tilting furnace. Instead, they simply function to attempt to stop flow through the tap hole when it is determined that most of the molten steel has been discharged through the tap hole and floating slag is beginning to flow through the tap hole. None of these prior art methods and apparatuses control or remove the slag from the flow after the flow has been discharged through the tap hole and into the trough.
It would be desirable to control slag in a tap discharge of molten metal after it flows through the tap hole into the trough and before it flows out of the trough and into the ladle.
It would also be beneficial if such an improved system could be effectively and readily employed on a tilting electric arc furnace having an attached discharge trough.
Additionally, such an improved system should provide for positive separation and control of the slag, including interface slag, from the molten metal.
Further, it would be desirable to provide an improved system which would permit the viewing of the level of molten metal and floating slag in the trough in order to coordinate the separation of the slag and metal, as well as the retention and discharge of the slag in a positive manner.
It would be beneficial to provide such a system which can be implemented by apparatus that can be removed and replaced as necessary, without requiring removal or replacement of the entire trough or furnace.
In particular, it would be desirable to provide a slag control system which can operate relatively efficiently and in a manner that will accommodate a relatively high flow rate in the tap discharge so as to minimize the total time required for the tap discharge (i.e., pour time). This would serve to reduce the amount of heat absorbed by the system, such as the refractory brick and steel supporting frame. This would also reduce the thermal cycle peaks and minimize the thermal degradation and wear of the materials.
In addition, a reduced tap discharge time can reduce the amount of gases absorbed by the molten metal in the trough, as well as in the ladle. In particular, it would be desirable to reduce the amount of nitrogen and oxygen absorbed by molten steel during tap discharges.
It would also be beneficial to provide a slag control system that would accommodate relatively high flow rates, and therefore reduce the total tap discharge time, so as to prevent an excessive temperature drop in the molten metal flowing along the trough as well as in the molten metal in the ladle. If high enough flow rates can be accommodated through an improved slag control system, the need to reheat the steel in the ladle may be eliminated or at least minimized.
For example, in one type of conventional electric arc furnace having a tap hole with a diameter of between 8 inches and 12 inches and having a conventional open trough, a tap discharge of 80 tons of molten slag and steel might require about 3 minutes. It would be desirable to provide an improved slag control system which, during operation, would not add significantly to the tap discharge time.
It would also be desirable to provide an improved slag control system which could readily accommodate designs employing sufficient thicknesses of refractory materials reduce heat transfer from the molten steel and to provide sufficiently rugged designs that can better withstand the effects of the hot flowing metal and high temperatures.
Further, it would be advantageous if an improved slag control system would be provided with the capability for eliminating or substantially minimizing irregularities in molten metal flow. Such flow irregularities are undesirable and can contribute to entraining slag into the molten metal through vortex effects or through other effects. The likelihood of entraining slag, or drawing interface slag, into the molten metal increases with time near the end of the tap discharge when the ratio of the steel to the slag in the total flow is relatively low. Of course, the inclusion of slag in the molten steel is undesirable for the reasons discussed in detail above.
Further, it would be desirable to provide an improved slag control system for controlling a discharge of molten metal and slag from a furnace wherein an apparatus for receiving the molten metal and slag is completely detached from the furnace and can be controlled and tilted on its own.
The present invention improvements are directed to minimizing the above-described problems, and the invention provides a number of operating improvements.